When a subscriber is registered at a foreign network, such as an IP (Internet Protocol) multimedia network, the home HSS (Home Subscriber Server) normally knows the address of the S-CSCF (Serving Call State Control Function) where the subscriber is registered Since it is desirable to hide the networks, except for the contact points which are usually I-CSCFs (Interrogating Call State Control Functions), the foreign network cannot give the name and/or address of the S-CSCF to the home HSS.
One proposed solution is that the HSS query should also be used in the visited network to locate the S-CSCF while another proposed solution is that the association between the subscriber and the name and/or address of the S-CSCF should be found from the I-CSCF. Locating the associations in the I-CSCF would require a new functionality in the I-CSCF. If the HSS contains the associations, it would result in the records of foreign subscribers being stored in the HSS. This would disturb the structure of the HSS in that it was designed to only store the records of its own subscribers. In addition, the HSS must also include the address of the APSE (Application Server). If the home operator does not want the address of the APSE available to other operators, a mechanism is needed to refer to the APSE.
Furthermore, in the visited network model, as noted above, the name and/or IP address of the S-CSCF are revealed during the registration to the home HSS when the S-CSCF requests the profile of the roaming subscriber from the home HSS of the subscriber. In addition, the IP address of the S-CSCF is also revealed in the originating and terminating call cases to/from new operators because the name, i.e. the FQDN (Fully Qualified Domain Name) of the S-CSCF has to be publicly resolvable.
In the home network model, the name and the IP address of the P-CSCF (Proxy CSCF, that is, the initial proxy in the home model) are revealed to the I-CSCF of the home operator during the registration when the P-CSCF sends a REGISTER message to the home I-CSCF of the roaming subscriber. The name and the IP address of the S-CSCF in the home network are revealed in originating and, depending on routing, possibly also in terminating call cases to new operators.
Lastly, in 3GPP IP multimedia network, there is an actual requirement to hide the internal structure of the network with respect to other networks. This implies that the names and the IP addresses of network elements, such as the S-CSCF shall not be made known to other networks.
It has been proposed that the HSS in a visited network behave as a VLS (Visited Location Server) to handle (that is, to select at registration and store for MT call routing) the identity of the S-CSCF in the visited network in order to mask the identity of the S-CSCFv (Serving CSCF in the visited network) to the home network. The VLS will be interrogated by the I-CSCFv (Interrogating CSCF in the visited network) when an MT (mobile terminating) call is routed to it by the home network. However, in such a solution, the visited network must maintain a relationship through some mechanism between the identity of the roaming subscriber and the HSSv (HSS in the visited network) that stores the identity of the S-CSCF. In addition, the relationship must be available to all I-CSCFv's since the I-CSCFv that receives the MT call routed from the home network cannot be decided nor predicted in advance.